A few organic free radicals are relatively stable. Three general methods are used to form free radicals. The three methods are thermal generation, photochemical generation, and redox generation. Radicals normally react in one of three ways: by abstracting an atom from a saturated bond; by adding to an unsaturated bond; and by reacting with other radicals, leading to either combination or disproportionation. Free radicals add to the common unsaturated groupings. Free-radical reactions may be divided into two classes. In the first, the product results from the combination of two radicals, as in the Kolbe synthesis. In the second class, the product results from the reaction of a radical with a molecule, as in the photochemical chlorination of methane. A large group of synthetically useful radical-catalyzed reactions is based on the addition of aliphatic carbon radicals to C=C bonds. Both alkenyl and aryl radicals can be generated by the tributylstannane method.